Many power circuits, such as DC-DC converters and DC/AC converters, use resonant power conversion. In the simplest configuration, electronic switches drive a resonant inductive-capacitive (L-C) tank circuit that is transformer coupled to a rectifier and a load. Typically, the L-C resonant circuit is excited at an operating frequency and various inefficiencies result if the L-C resonant frequency is different than a specific desired value. Various ways of actively tuning a resonant circuit are possible, but tuning capability typically adds complexity and cost.
One example system that typically uses resonant power conversion is wireless power transfer. Wireless power transfer is useful for charging batteries. For example, wireless charging holders (or docks, or mats) exist for electric toothbrushes, cell phones and other consumer electronics, construction equipment (such as saws and drills), and industrial scientific medical (ISM) devices. Typically, a power transmitting device (source) includes a primary transformer coil, a power receiving device includes a secondary transformer coil, and the two coils are placed in close proximity for charging. Power is inductively coupled from the power transmitter to the power receiver through the resulting transformer. Multiple coils in the power transmitter and/or power receiver may facilitate X and Y spatial freedom. One power transmitter may provide power for multiple power receivers.
ISM devices commonly use standard RF frequencies that are dedicated to ISM, with strict out-of-band emission requirements. Accordingly, ISM wireless power transmitters and receivers commonly operate at a fixed frequency. Typically, ISM wireless power transmitters include a DC/DC converter that drives a DC/AC converter, where the DC-AC converter output is at a constant standard ISM frequency. Power is controlled by controlling the output of the DC-DC converter in the power transmitter.
Wireless power transfer systems may also include wireless communication from the power receiver to the power transmitter. For example, the power receiver may need to communicate battery level, or may need to communicate a need for trickle charging, or deep cycle recharging. Wireless power receivers may include separate wireless communication ability (such as, Bluetooth low energy (BLE)), and wireless power transmitters may also include compatible separate wireless communication ability to receive information from the power receivers.
FIG. 1 shows an example of a conventional wireless power transfer system 100, which includes a power transmitter 102 and a power receiver 104. The power transmitter 102 includes a primary coil 106 and the power receiver 104 includes a secondary coil 108.
The example power transmitter 102 includes a DC/DC converter 110 and a DC/AC converter 112. The DC/AC converter 112 drives the primary coil 106 at a frequency that may be variable or fixed, and in particular may be a fixed standard ISM frequency. A controller 114 controls power output by controlling the output of the DC/DC converter 110.
The example power receiver 104 includes a rectifier 116, a charger 118, and a battery 120. Typically, the rectifier 116 is a diode full-wave rectifier and the charger 118 is a DC/DC converter, such as a buck converter (whose output voltage is lower than its input voltage). The example power receiver 104 also includes a BLE communication device 122, which wirelessly transmits control information to a BLE communication device 124 in the power transmitter 102.